Time-of-Flight (TOF) measures the time that it takes for an object, particle or acoustic/electromagnetic to travel a distance through a medium. This measurement can be used as a way to measure velocity or path-length through the given medium, or as a way to learn about the particle or the medium (such as composition or flow rate). The traveling object may be detected directly (e.g., ion detector in mass spectrometry) or indirectly (e.g., light scattered from an object in laser Doppler velocimetry).
In ultrasonic flow meter measurement, in order to estimate total flow velocity, TOF is used to measure the speed of a signal propagating upstream, and downstream, of flow of a media. This measurement is made in a collinear direction with the flow. In planar Doppler velocimetry (optical flow meter measurement), TOF measurements are made perpendicular to the flow by timing when individual particles cross two or more locations along the flow. In optical interferometry, the path-length difference between sample and reference arms can be measured by TOF methods, such as frequency modulation followed by phase shift measurement. Such methods are used in laser radar and laser tracker systems for medium to long range distance measurement.
Refer to FIG. 1A, an exemplary sensor system has a transmitter 110 and a receiver 120. A target 130 is located in front of the sensor by certain distance. The sensor system is used to measure this distance between the sensor and the target. An acoustic or electromagnetic wave 111 is transmitted from the transmitter. The wave is reflected by the target. Some portion of the reflected wave 121 is received by the receiver. The TOF is the time elapsed between the moment that the wave is being transmitted and the moment of the wave being received. The distance can be calculated from the measured TOF since the speed of the wave is known.
In FIG. 1B, an electric circuit called Time-to-Digital Converter (TDC) is used to measure the time elapsed between the transits of signals Start and Stop. Signal Start is delayed by a plurality of n delay lines. The delay lines have time delays of t0+σ, t0+2·σ, t0+3·σ, . . . , t0+n·σ, respectively. The ends of the delay lines are connected to the data inputs of a plurality of n flip-flops. The flip-flops are clocked by the Stop signal. The outputs of the flip-flops are fed to a decoder circuit. The value represented by these outputs is the time duration between Start and Stop. The time resolution is σ. In TOF measurement discussed in FIG. 1A, the Start signal represents the moment that the transmitter starts to transmit the wave. The Stop signal is the moment of the wave reaching the receiver.
FIG. 1C is another type of TDC where the Start signal is passed though a delay line made of a series of identical delay cells. The time delay associated with each delay cell is σ. Signals are tapped out from each of those delay cells and fed to a group of flip-flops. The flip-flops are clocked by the Stop signal. The outputs of the flip-flops are fed to a decoder whose output value represents the TOF between Start and Stop. The time resolution is σ. FIG. 1D illustrates a third type of TDC where both the Start and Stop signals are passed through delay lines. The two delay lines are however made of delay cells of slightly different time delays, σ1 and σ2, respectively. Thus the time resolution is improved to σ1−σ2, assuming σ1>σ2.
One problem with the method of using TDC to measure TOF is that the absolution value of the TOF measurement is difficult to be known in high accuracy. To get high accuracy on TOF measurement, the precise delay values of σ, σ1, σ2 must be known. This is difficult unless the delay line is locked to a known frequency by PLL or DLL for the cases of FIG. 1C and FIG. 1D. For the case of FIG. 1B, high accuracy is virtually impossible since the delay value depends on the capacitor C's capacitance value which could varies in large degree under different PVT (process, voltage, temperature) condition. For these reasons, TDC is not good for TOF measurement. However, they are useful to detect the relative phase different between two signals. This is especially applicable for phase detector in Phase Locked Loop design.
As discussed previously, the time resolution of TDC is limited by cell delay a. It can be improved by the method of using delay difference of two delay cells: σ1−σ2. The principle of using the delay difference approach to measure TOF can be illustrated by the example of using Vernier caliper to measure distance. Refer now to FIG. 2, a Vernier caliper is used to measure distance. It has two scales: the main scale 210 and the Vernier scale 220. The Vernier scale is constructed in such a way that its graduations are at a slightly smaller spacing than those on the main scale. When the zero point of the Vernier scale is coincident with the start 230 of the main scale, none but the last graduation in Vernier scale coincides with a graduation 240 on the main scale. Therefore, N graduations of the Vernier scale cover N−1 graduations of the main scale. N is defined in this application as “Vernier factor”. In Vernier scale, its graduation is spaced at a constant fraction of that of main scale. For the Vernier caliper displayed in FIG. 2, N=50. Thus, the measurement resolution is 1 mm/50=0.02 mm.
For an example of N=10, the marks on the Vernier scale is spaced nine tenths of those on the main scale. If the two scales are put together with zero points aligned, the first mark on the Vernier scale is one tenth short of the first main scale mark, the second two tenths short, and so on up to the ninth mark which is misaligned by nine tenths. Only when a full ten marks are counted is there an alignment since the tenth mark is ten tenths (a whole main scale unit short). At this time, the tenth mark on Vernier scale aligns with the ninth mark on the main scale. Now if the Vernier is moved by a small amount, for example one tenth of the main scale, the only pair of marks that come into alignment are the first pair since these were the only ones originally misaligned by one tenth. If it is moved by two tenths, the second pair aligns since these are the only ones originally misaligned by that amount. If it is moved by five tenths, the fifth pair aligns; and so on. For any movement, only one pair of marks aligns and that pair shows the value between the marks on the main scale.
Refer now to FIG. 3, the Vernier method is applied to build an electronic system 300 of measuring TOF. System 300 has two oscillators: a slow oscillator 330 and a fast oscillator 340 with oscillation frequencies f1 and f2, respectively. Their output signals, slow clock 331 and fast clock 341, are fed to a phase detector 350. System 300 has two digital counters: counter#1 360 and counter#2 370. The counters are driven by signals slow clock 331 and fast clock 341, respectively. When the edges of slow clock and fast clock are aligned, a point-of-coincidence is reached. At this moment, the phase detector 350 generates a signal Reset 351 that is used to read out the counters' contents, n1 361 and n2 371, and then reset the counter#1 360 and counter#2 370.
System 300 is used to measure the time elapsed between signals Start 310 and Stop 320. The time elapsed is the TOF whose value is τ 380. Signal Start 310 is used to enable the slow oscillator 330 and signal Stop is used to enable the fast oscillator 340. FIG. 4 shows exemplary waveforms. As shown, the slow clock 430 with period T1 is enabled by Start 410 and the fast clock 440 with period T2 is enabled by Stop 420. The TOF is the time difference between Start 410 and Stop 420. The value of TOF is τ 460. When the point-of-coincidence 450 is reached, equation (1) is established and TOF value τ can be calculated.τ+n2·T2=n1·T1τ=n1·T1−n2·T2=n1/f1−n2/f2  (1)τ=(n1−n2)/(f1/df)+n1·df/[f1·(f1+df)]=(n1−n2)/f2+n1·df/(f1·f2   (2)
If the fast clock's frequency f2 is expressed as f2=f1+df, the TOF value τ can be derived as in (2). From (2), it can be seen that high measurement resolution on τ can be achieved by small df. Similar to the case of Vernier caliper where the larger the Vernier factor N is, the higher the measurement resolution will be. In the case of system of 300, the smaller the df is, the higher the measurement resolution will be. The key requirement for system 300 is therefore the frequency generation for the slow and fast oscillators. Their frequencies must be generated in high accuracy and the frequency granularity of the frequency generators must be made as small as possible.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.